A hard disk drive (HDD) that is a type of a magnetic recording and reproducing device is currently increased in recording density at a rate of 50% or more a year, and it is said that this trend will continue in the future. According to this trend, development of a magnetic head and a magnetic recording medium which are suitable for high recording densification is in progress.
At present, a magnetic recording medium that is mounted on a commercially available magnetic recording and reproducing device is a so-called vertical magnetic recording medium in which the axis of easy magnetization in a magnetic film is primary oriented vertically. A vertical magnetic recording medium is reduced in the influence of a diamagnetic field at a boundary region between recording bits even with high recording densification, and therefore, a clear bit boundary is formed. Accordingly, an increase in noise is suppressed. Furthermore, a reduction in recording bit volume along with high recording densification is small, the vertical magnetic recording medium is also resistant to the heat fluctuation effect. In light of this, recently, a magnetic recording medium has attracted attention, and there is proposed a structure for a medium suitable for the vertical magnetic recording.
In addition, recent studies have been directed to the use of a single-pole head having high ability for writing to the vertical magnetic layer so as to cope with a demand for high recording densification of a magnetic recording medium. To correspond to such a single-pole head, there is suggested a magnetic recording medium in which a layer, which is called a backing layer and is formed from a soft magnetic material, is provided between a vertical magnetic layer that is a recording layer and a non-magnetic substrate so as to improve magnetic flux entry and exit efficiency between the single-pole head and the magnetic recording medium.
However, in a magnetic recording and reproducing device using a magnetic recording medium in which the above-described backing layer is simply provided, recording and reproducing characteristics, heat fluctuation resistance, or magnetic resolution during recording and reproduction may not be satisfactory, and thus a magnetic recording medium excellent in these characteristics is desired.
Particularly, a high S/N ratio in which the ratio of reproduced signal to noise (S/N ratio) is increased, and improvement in heat fluctuation resistance are important as recording and reproducing characteristics; and in the future, these two items will be necessary for high recording densification. However, these two items are in a trade-off relationship, and thus when one of these items is improved, the other is degraded, and thus achieving a high level of both these items is a primary problem.
To solve the problem, there is suggested a magnetic recording medium in which three magnetic layers are joined by AFC (Anti Ferro Coupling) using a non-magnetic layer or the like, and thus a decrease in the S/N ratio is not caused while enjoying an advantage of a decrease in synthetic Mrt and PW50 (for example, refer to Patent Document 1).
Specifically, Patent Document 1 discloses a magnetic recording medium provided with a substrate, a first lower ferromagnetic layer which is disposed on the substrate and has a residual magnetization Mr, a thickness t, and a product Mrt of a residual magnetization and thickness, a ferromagnetic coupling layer disposed on the first lower ferromagnetic layer, a second lower ferromagnetic layer which is disposed on the ferromagnetic coupling layer and has an Mrt value, an antiferromagnetic coupling layer disposed on the second lower ferromagnetic layer, and an upper ferromagnetic layer which is disposed on the antiferromagnetic coupling layer, and which has an Mrt value larger than the sum of the Mrt values of the first and second lower ferromagnetic layers.
On the other hand, to improve the recording and reproducing characteristics and the heat fluctuation characteristics of the vertical magnetic recording medium, there is suggested a technology in which multi-layered magnetic layers are formed using an orientation control layer, and crystal grains of the respective magnetic layers are made into continuous columnar crystals, thereby increasing the vertical orientation of the magnetic layers (for example, refer to Patent Document 2).
In addition, there is disclosed a technology in which as an orientation control layer, for example, Ru is used. Ru has a dome-shaped convex portion at an apex portion of the columnar crystals. Accordingly, Ru has an effect in that crystal grains of the magnetic layer or the like are allowed to grow on the convex portion, a separation structure of the grown crystal grains is promoted, the crystal grains are isolated, and magnetic particles are grown in a columnar shape (for example, refer to Patent Document 3).
In addition, there is suggested a technology in which a NiW alloy layer as a seed layer is provided between a soft magnetic layer and an orientation control layer for refinement of the columnar crystal that constitutes the orientation control layer (For example, refer to Patent Document 4).
In addition, there is suggested a magnetic recording medium having a structure in which a soft magnetic layer, an intermediate layer, a vertical recording layer, and a protective layer are provided on a substrate. In the magnetic recording medium, from a side close to a substrate, a first intermediate layer is a metal layer containing at least one kind of element selected from a group consisting of Pd, Pt, Au, Ag, Rh, Ru, and Ti as a main component, a second intermediate layer is an oxygen-containing layer, a third intermediate layer is a metal layer containing at least one kind of element selected from a group consisting of Pd, Pt, Au, Ag, Rh, Ru, and Ti as a main component, a fourth intermediate layer is a metal layer containing at least one kind of element selected from Co, Cr, Ru, and Ti as a main component, and the vertical recording layer contains Co as a main component and also contains oxygen (for example, refer to Patent Document 5).
In addition, paragraphs [0010] to [0011] of Patent Document 5 disclose that oxygen is allowed to be contained in a recording layer using a four-layered intermediate layer. In the four-layered intermediated layer, vertical orientation is controlled by the first intermediate layer, an island-shaped isolated structure (convex structure) is formed in the oxygen-containing layer that is the second intermediate layer, and the metal layer that is the third intermediate layer, the fourth intermediate layer having a hexagonal close packing structure is further formed using the convex portion as a nucleus. According to this, a coercive force and a squareness ratio of the recording layer are improved, a media-noise is reduced, and heat-resistant demagnetization characteristics are improved.